Micromotor

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Micromotors are very small particles (measured in microns) that can move themselves. [1] The term is often used interchangeably with "nanomotor," despite the implicit size difference. These micromotors actually propel themselves in a specific direction autonomously when placed in a chemical solution. There are many different micromotor types operating under a host of mechanisms. Easily the most important examples are biological motors such as bacteria and any other self-propelled cells. Synthetically, researchers have exploited oxidation-reduction reactions to produce chemical gradients, local fluid flows, or streams of bubbles that then propel these micromotors through chemical media. [2] Different stimuli, both external (light, [3] magnetism [4] ) and internal (fuel concentration, material composition, [5] particle asymmetry [6] ) can be used to control the behavior of these micromotors.

Contents

Micromotors may have applications in medicine since they have been shown to be able to deliver materials to living cells within an organism. They also have been shown to be effective in degrading certain chemical and biological warfare agents.

Janus Motor Propulsion

Janus particle micromotors consist of two or more components with distinct physical properties, such as a titanium dioxide particle capped with gold, [7] or a polystyrene bead coated on one side with a layer of platinum [8] which both display a difference in catalytic activity between halves. When these motors are placed in a fuel, such as hydrogen peroxide, one redox half-reaction occurs on each pole according to catalytic activity. As the oxidation reaction produces electrons and protons, the reduction reaction consumes these as reactants on the opposite pole of the particle, this movement of molecules generates a fluid flow across the surface of the motor and this drives the particle forward. The catalytic difference between each pole of the Janus motor can be characteristic of the material [9] such as metals which catalyze at different rates, or induced by external stimuli like UV light [7] which can be absorbed by semi-conductor materials like titanium dioxide to excite electrons for the redox reaction.

Catalytic activity is not the only way to generate motion using Janus materials; self-propelled Janus droplets can be made using a complex emulsion of two different surfactant oils [10] which move forward spontaneously due to the difference in surface tension as the two oils solubilize.

Nano particle Implementation

Nano particle incorporation into micromotors has been recently studied and observed further. Specifically, gold nanoparticles have been introduced to the traditional titanium dioxide outer layer of most micromotors. [11] The size of these gold nanoparticles typically is distributed from anywhere around 3 nm to 30 nm. [12] Since these gold nanoparticles are layered on top of the inner core (usually a reducing agent, such as magnesium), there is enhanced macrogalvanic corrosion observed. [13] Technically, this is where the cathode and anode are in contact with each other, creating a circuit. The cathode, as a result of the circuit, is corroded. The depletion of this inner core leads to the reduction of the chemical environment as a fuel source. For example, in a TiO2/Au/Mg micromotor in a seawater environment, the magnesium inner core would experience corrosion and reduce water to begin a chain of reactions that results in hydrogen gas as a fuel source. The reduction reaction is as follows: [11]

Applications

Researchers hope that micromotors will be used in medicine to deliver medication and do other precise small-scale interventions. [14] A study has shown that micromotors could deliver gold particles to the stomach layer of living mice. [15]

Photocatalytic Degradation of Biological and Chemical Warfare Agents

Micromotors are capable of photocatalytic degradation with the appropriate composition. [16] [17] Specifically, micromotors with a titanium dioxide/gold nanoparticle outer layer and magnesium inner core are currently being examined and studied for their degradation efficacy against chemical and biological warfare agents (CBWA). These new TiO2/Au/Mg micromotors produce no reagents or toxic byproducts from the propulsion and degradation mechanisms. However, they are very effective against CBWAs and present a complete and rapid degradation of certain CBWAs. There has been recent research of TiO2/Au/Mg micromotors and their use and degradation efficacy against biological warfare agents, such as Bacillus anthracis, and chemical warfare agents, such as organophosphate nerve agents- a class of acetylcholinesterase inhibitors. Therefore, application of these micromotors is a possibility for medical and environmental applications.

Photocatalytic Degradation Mechanism

These new micromotors are composed of a photoactive photocatalyst outer/surface layer that often has active metal nanoparticles (platinum, gold, silver, etc.) on the surface as well. [18] Under UV irradiation, the adsorbed water produces strongly oxidizing hydroxyl radicals. Also, adsorbed molecular O2 reacts with electrons producing superoxide anions. Those superoxide anions also produce to the production of peroxide radicals, hydroxyl radicals, and hydroxyl anions. Transformation into carbon dioxide and water, otherwise known as mineralization, of CWAs has been observed as a result of the radicals and anions. Also, the active metal nanoparticles effectively shift the Fermi level of the photocatalyst, enhancing the distribution of the electron charge. Therefore, the lifetime of the radicals and anions is extended, so the implementation of the active metal nanoparticles has greatly improved photocatalytic efficiency.

Metal-Organic Framework (MOF) based Micromotors

Metal–organic frameworks (MOFs) are a class of compounds that are composed of a metal ion cluster coordinated to an organic linker. These compounds can form 1D, 2D and 3D structures. They possess a porous morphology which can be tuned in terms of shape and size depending on the metal ion and organic linker used to form the MOF. These pores grants them great catalytic properties which is why MOF research focused on the catalytic degradation of contaminants for environmental remediation has been gaining more attention. The major limitation of MOFs is that they tend to settle at the bottom of the solution, reducing their effectiveness since they are not coming into contact with the contaminant. Thus, in the past years more and more research focused on MOF for catalytic degradation have been implementing micromotors. The MOF particles are half-coated with a metal, creating a Janus motor particle (half metal, half MOF). The motor aspect of the particle enhances its diffusion, increasing the probability of the MOF and contaminant encountering each other in solution, thus increasing its degradation rate. These MOF based micromotors have proven to be extremely efficient at decontaminating water, and after the fuel used for propulsion (in most cases hydrogen peroxide) is completely consumed, they settle at the bottom of the solution, facilitating the removal of the Janus motor particles from the solution. [19] [20]

Related Research Articles

<span class="mw-page-title-main">Titanium dioxide</span> Chemical compound often used as a white pigment, Including in food and paints.

Titanium dioxide, also known as titanium(IV) oxide or titania, is the inorganic compound with the chemical formula TiO
2. When used as a pigment, it is called titanium white, Pigment White 6 (PW6), or CI 77891. It is a white solid that is insoluble in water, although mineral forms can appear black. As a pigment, it has a wide range of applications, including paint, sunscreen, and food coloring. When used as a food coloring, it has E number E171. World production in 2014 exceeded 9 million tonnes. It has been estimated that titanium dioxide is used in two-thirds of all pigments, and pigments based on the oxide have been valued at a price of $13.2 billion.

A "photoelectrochemical cell" is one of two distinct classes of device. The first produces electrical energy similarly to a dye-sensitized photovoltaic cell, which meets the standard definition of a photovoltaic cell. The second is a photoelectrolytic cell, that is, a device which uses light incident on a photosensitizer, semiconductor, or aqueous metal immersed in an electrolytic solution to directly cause a chemical reaction, for example to produce hydrogen via the electrolysis of water.

<span class="mw-page-title-main">Nanoparticle</span> Particle with size less than 100 nm

A nanoparticle or ultrafine particle is usually defined as a particle of matter that is between 1 and 100 nanometres (nm) in diameter. The term is sometimes used for larger particles, up to 500 nm, or fibers and tubes that are less than 100 nm in only two directions. At the lowest range, metal particles smaller than 1 nm are usually called atom clusters instead.

<span class="mw-page-title-main">Nanomotor</span> Molecular device capable of converting energy into movement

A nanomotor is a molecular or nanoscale device capable of converting energy into movement. It can typically generate forces on the order of piconewtons.

<span class="mw-page-title-main">Photocatalysis</span> Acceleration of a photoreaction in the presence of a catalyst

In chemistry, photocatalysis is the acceleration of a photoreaction in the presence of a photocatalyst, the excited state of which "repeatedly interacts with the reaction partners forming reaction intermediates and regenerates itself after each cycle of such interactions." In many cases, the catalyst is a solid that upon irradiation with UV- or visible light generates electron–hole pairs that generate free radicals. Photocatalysts belong to three main groups; heterogeneous, homogeneous, and plasmonic antenna-reactor catalysts. The use of each catalysts depends on the preferred application and required catalysis reaction.

<span class="mw-page-title-main">Cerium(IV) oxide</span> Chemical compound

Cerium(IV) oxide, also known as ceric oxide, ceric dioxide, ceria, cerium oxide or cerium dioxide, is an oxide of the rare-earth metal cerium. It is a pale yellow-white powder with the chemical formula CeO2. It is an important commercial product and an intermediate in the purification of the element from the ores. The distinctive property of this material is its reversible conversion to a non-stoichiometric oxide.

Nanomaterial-based catalysts are usually heterogeneous catalysts broken up into metal nanoparticles in order to enhance the catalytic process. Metal nanoparticles have high surface area, which can increase catalytic activity. Nanoparticle catalysts can be easily separated and recycled. They are typically used under mild conditions to prevent decomposition of the nanoparticles.

As the world's energy demand continues to grow, the development of more efficient and sustainable technologies for generating and storing energy is becoming increasingly important. According to Dr. Wade Adams from Rice University, energy will be the most pressing problem facing humanity in the next 50 years and nanotechnology has potential to solve this issue. Nanotechnology, a relatively new field of science and engineering, has shown promise to have a significant impact on the energy industry. Nanotechnology is defined as any technology that contains particles with one dimension under 100 nanometers in length. For scale, a single virus particle is about 100 nanometers wide.

An electroosmotic pump (EOP), or EO pump, is used for generating flow or pressure by use of an electric field. One application of this is removing liquid flooding water from channels and gas diffusion layers and direct hydration of the proton exchange membrane in the membrane electrode assembly (MEA) of the proton exchange membrane fuel cells.

<span class="mw-page-title-main">Janus particles</span> Type of nanoparticle or microparticle

Janus particles are special types of nanoparticles or microparticles whose surfaces have two or more distinct physical properties. This unique surface of Janus particles allows two different types of chemistry to occur on the same particle. The simplest case of a Janus particle is achieved by dividing the particle into two distinct parts, each of them either made of a different material, or bearing different functional groups. For example, a Janus particle may have one half of its surface composed of hydrophilic groups and the other half hydrophobic groups, the particles might have two surfaces of different color, fluorescence, or magnetic properties. This gives these particles unique properties related to their asymmetric structure and/or functionalization.

<span class="mw-page-title-main">Platinum nanoparticle</span>

Platinum nanoparticles are usually in the form of a suspension or colloid of nanoparticles of platinum in a fluid, usually water. A colloid is technically defined as a stable dispersion of particles in a fluid medium.

<span class="mw-page-title-main">Self-propelled particles</span> Type of autonomous agent

Self-propelled particles (SPP), also referred to as self-driven particles, are terms used by physicists to describe autonomous agents, which convert energy from the environment into directed or persistent motion. Natural systems which have inspired the study and design of these particles include walking, swimming or flying animals. Other biological systems include bacteria, cells, algae and other micro-organisms. Generally, self-propelled particles often refer to artificial systems such as robots or specifically designed particles such as swimming Janus colloids, bimetallic nanorods, nanomotors and walking grains. In the case of directed propulsion, which is driven by a chemical gradient, this is referred to as chemotaxis, observed in biological systems, e.g. bacteria quorum sensing and ant pheromone detection, and in synthetic systems, e.g. enzyme molecule chemotaxis and enzyme powered hard and soft particles.

Nanoremediation is the use of nanoparticles for environmental remediation. It is being explored to treat ground water, wastewater, soil, sediment, or other contaminated environmental materials. Nanoremediation is an emerging industry; by 2009, nanoremediation technologies had been documented in at least 44 cleanup sites around the world, predominantly in the United States. In Europe, nanoremediation is being investigated by the EC funded NanoRem Project. A report produced by the NanoRem consortium has identified around 70 nanoremediation projects worldwide at pilot or full scale. During nanoremediation, a nanoparticle agent must be brought into contact with the target contaminant under conditions that allow a detoxifying or immobilizing reaction. This process typically involves a pump-and-treat process or in situ application.

Many experimental realizations of self-propelled particles exhibit a strong tendency to aggregate and form clusters, whose dynamics are much richer than those of passive colloids. These aggregates of particles form for a variety of reasons, from chemical gradients to magnetic and ultrasonic fields. Self-propelled enzyme motors and synthetic nanomotors also exhibit clustering effects in the form of chemotaxis. Chemotaxis is a form of collective motion of biological or non-biological particles toward a fuel source or away from a threat, as observed experimentally in enzyme diffusion and also synthetic chemotaxis or phototaxis. In addition to irreversible schooling, self-propelled particles also display reversible collective motion, such as predator–prey behavior and oscillatory clustering and dispersion.

Collective motion is defined as the spontaneous emergence of ordered movement in a system consisting of many self-propelled agents. It can be observed in everyday life, for example in flocks of birds, schools of fish, herds of animals and also in crowds and car traffic. It also appears at the microscopic level: in colonies of bacteria, motility assays and artificial self-propelled particles. The scientific community is trying to understand the universality of this phenomenon. In particular it is intensively investigated in statistical physics and in the field of active matter. Experiments on animals, biological and synthesized self-propelled particles, simulations and theories are conducted in parallel to study these phenomena. One of the most famous models that describes such behavior is the Vicsek model introduced by Tamás Vicsek et al. in 1995.

<span class="mw-page-title-main">Copper nanoparticle</span>

A copper nanoparticle is a copper based particle 1 to 100 nm in size. Like many other forms of nanoparticles, a copper nanoparticle can be prepared by natural processes or through chemical synthesis. These nanoparticles are of particular interest due to their historical application as coloring agents and the biomedical as well as the antimicrobial ones.

<span class="mw-page-title-main">Titanium dioxide nanoparticle</span>

Titanium dioxide nanoparticles, also called ultrafine titanium dioxide or nanocrystalline titanium dioxide or microcrystalline titanium dioxide, are particles of titanium dioxide with diameters less than 100 nm. Ultrafine TiO2 is used in sunscreens due to its ability to block ultraviolet radiation while remaining transparent on the skin. It is in rutile crystal structure and coated with silica or/and alumina to prevent photocatalytic phenomena. The health risks of ultrafine TiO2 from dermal exposure on intact skin are considered extremely low, and it is considered safer than other substances used for ultraviolet protection.

Jeffrey I. Zink is an American molecular biologist and chemist currently a Distinguished Professor at University of California, Los Angeles whose interests are in materials, nanoscience, physical and inorganic chemistry. His current research is examining molecules containing metal and nanomaterials. He worked with Fraser Stoddart to help develop machines that could be applied to deliver drugs. According to Google Scholar, his highest citations are 2,503, 2,131, 1,968, 1,873, and 1,150.

<span class="mw-page-title-main">Heterogeneous gold catalysis</span>

Heterogeneous gold catalysis refers to the use of elemental gold as a heterogeneous catalyst. As in most heterogeneous catalysis, the metal is typically supported on metal oxide. Furthermore, as seen in other heterogeneous catalysts, activity increases with a decreasing diameter of supported gold clusters. Several industrially relevant processes are also observed such as H2 activation, Water-gas shift reaction, and hydrogenation. One or two gold-catalyzed reactions may have been commercialized.

<span class="mw-page-title-main">Microswimmer</span>

A microswimmer is a microscopic object with the ability to move in a fluid environment. Natural microswimmers are found everywhere in the natural world as biological microorganisms, such as bacteria, archaea, protists, sperm and microanimals. Since the turn of the millennium there has been increasing interest in manufacturing synthetic and biohybrid microswimmers. Although only two decades have passed since their emergence, they have already shown promise for various biomedical and environmental applications.

References

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